Friday, 10 December 2010

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Question:“In HPLC, how can I calculate dwell volume for my system, and what is the main difference between dead volume and dwell volume?”

Answer:“The dwell volume for an HPLC system refers to the volume in the system from the point where the mobile phase is mixed to the inlet of the HPLC column. This volume is important when using gradient elution since there will be a delay between the time when a change in mobile phase is applied and when it actually reaches the column. The size of the dwell volume varies between different systems; it can be calculated as follows:

Remove the column from the system and use a short length of 0.010″ tubing to connect the injector directly to the detector.

For solvent A, use HPLC grade water; for solvent B, add about 0.1% acetone to water (methanol or acetonitrile can be used instead of water).

Set the detector wavelength to 265nm.

Run a typical gradient from 0 to 100% B (e.g. 0-100% in 20 minutes at 3 mL/min flow). Record the detector signal during this gradient.

Print out or display the "chromatogram" from the gradient run. It should look like Figure 1. Draw the best straight line fit to the flat portion at the beginning of the plot. Draw the best straight line fit to the linear ramp of the gradient. The time at which these two lines intersect is the dwell time (tD). The dwell volume is the product of the dwell time and the flow rate used for the test.

Figure 1

The term ‘system dead volume’ is used to refer to the volume in a HPLC system from the point of injection to that of detection. Think of it as the volume of mobile phase between these points. The ‘column dead volume’ is the volume of space in the column not taken up by stationary phase (sometimes also referred to as void volume), or you can also think of it as the volume of mobile phase that fits in the column. Typically the difference between the system dead volume and column dead volume is very small and so you may find the terms being used interchangeably. Unfortunately chromatographers can sometimes be a bit sloppy about using consistent terminology but the context should make it clear what is being referred to.”